1. Field of the Invention
The present invention relates to a surveying instrument which incorporates a magnetic incremental rotary encoder suitable for surveying instruments such as total stations theodolites or the like.
2. Description of the Related Art
Some conventional surveying instruments such as total stations, theodolites or the like are provided with an incremental rotary encoder as an angle measuring device. Conventionally, an optical incremental rotary encoder is frequently used because it has a high degree of stability and precision.
Similar to an optical incremental rotary encoder, a magnetic incremental rotary encoder is also known as an angle measuring device. A magnetic incremental rotary encoder is generally provided with a magnetic drum (graduator disc) and a magnetic sensor. The magnetic incremental rotary encoder is provided, on an outer peripheral surface of the magnetic drum thereof, with a multi-pole magnetized layer having a plurality of magnetized divisions equally divided by a number of divisions p (xe2x80x9cpxe2x80x9d being a positive integer). The magnetic sensor is positioned so as to face the multi-pole magnetized layer. This magnetic sensor is provided thereon with, e.g., four magnetoresistor elements which are disposed at equally spaced intervals, whose pitch is smaller than that of the plurality of magnetized divisions of the multi-pole magnetized layer, to detect the variation in the resistance values of the four magnetoresistor elements which vary in accordance with the rotation of the magnetic drum to thereby determine the rotational angle of the magnetic drum with high precision corresponding to the pitch of the plurality of magnetized divisions of the multi-pole magnetized layer. An angle smaller than the pitch is determined according to an interpolative calculation. Since surveying instruments are generally required to have a high degree of precision in their functions, it is desirable to decrease the pitch of the plurality of magnetized divisions of the multi-pole magnetized layer by increasing the number of the magnetized divisions of the multi-pole magnetized layer, so that the number of magnetoresistor elements can be increased.
In magnetic incremental rotary encoders, noise due to a Barkhausen effect occurs. This noise (hereinafter referred to as xe2x80x9cBarkhausen noisexe2x80x9d) is a minute-amplitude noise in a magnetization curve (B-H curve) that occurs with abrupt changes in magnetization occurring due to the movement of the multi-pole magnetized layer (i.e., the rotation of the magnetic drum) within a range wherein a large change in intensity of magnetization occurs in the magnetization curve obtained when a magnetic field is applied to a ferromagnetic medium. As the pitch of the plurality of magnetized divisions of the multi-pole magnetized layer is decreased to increase the number of magnetoresistor elements, Barkhausen noise tends to occur more, which increases an influence on the encoder.
An object of the present invention is to provide a surveying instrument which incorporates a magnetic incremental rotary encoder and which makes it possible to reduce factors such as Barkhausen noise, which cause detection error, so as to maintain sufficient precision.
To achieve the object mentioned above, according to the present invention, a surveying instrument is provided, which incorporates a magnetic incremental rotary encoder, including a rotary magnetic drum provided on an outer peripheral surface thereof with a multi-pole magnetized layer having a plurality of equally-divided magnetized divisions, at least one magnetic sensor located so as to face the multi-pole magnetized layer, and at least one magnet, positioned in the vicinity of the magnetic sensor, for reducing an error in detection of the magnetic incremental rotary encoder.
Preferably, the at least one magnetic sensor includes a plurality of magnetoresistor elements which are disposed at equally spaced intervals whose pitch is smaller than that of the plurality of equally-divided magnetized divisions.
The above-mentioned error is caused by a noise due to a Barkhausen effect.
In an embodiment, the at least one magnet is a permanent magnet.
Preferably, the at least one magnet is designed and positioned so that primary lines of magnetic force develop substantially parallel to a sensor surface of a corresponding magnetic sensor, and substantially orthogonal to lines of magnetic field of the multi-pole magnetized layer.
In an embodiment, the at least one magnet includes a sheet magnet positioned on the opposite side of a corresponding magnetic sensor with respect to the multi-pole magnetized layer.
In an embodiment, the at least one magnet includes a pair of sheet magnets positioned on a sensor surface of the magnetic sensor on opposite sides of the plurality of magnetoresistor elements.
In an embodiment, the at least one magnet is fixed to a stationary member to which at least one bearing for supporting the rotary magnetic drum is fixed.
In an embodiment, the at least one magnet is fixed to a cover which is attached to a stationary member to which at least one bearing for supporting the rotary magnetic drum is fixed.
In another embodiment, at least one magnet includes a first magnet fixed to a stationary member to which at least one bearing for supporting the rotary magnetic drum is fixed, and a second magnet fixed to a cover attached to the stationary member. The first magnet and the second magnet are positioned on opposite sides of the magnetic sensor.
The surveying instrument can be a total station.
Preferably, the at least one magnetic sensor includes a first magnetic sensor and a second magnetic sensor. The first magnetic sensor and said second magnetic sensor are is provided in the magnetic incremental rotary encoder so that a phase difference of the one magnetic sensor with respect to the other of the pair of magnetic sensors is adjustable.
According to another aspect of the present invention, a surveying instrument is provided, including a leveling board, a pedestal coupled to the leveling board to be rotatable about a vertical axis relative to the leveling board, a collimating telescope coupled to the pedestal to be rotatable about a horizontal axis relative to the pedestal, a horizontal-angle measuring device for measuring an angle of rotation of the pedestal relative to the leveling board, and a vertical-angle measuring device for measuring an angle of rotation of the collimating telescope relative to the pedestal. At least one of the horizontal-angle measuring device and the vertical-angle measuring device includes a magnetic incremental rotary encoder. The magnetic incremental rotary encoder includes a rotary magnetic drum provided on an outer peripheral surface thereof with a multi-pole magnetized layer having a plurality of equally-divided magnetized divisions, a magnetic sensor; and at least one magnet for reducing an error in detection of the magnetic incremental rotary encoder. The magnetic sensor includes a plurality of magnetoresistor elements and is provided so as to face the multi-pole magnetized layer. Furthermore, at least one magnet is positioned in the vicinity of the magnetic sensors.
The present disclosure relates to subject matter contained in Japanese Patent Application No.11-301116 (filed on Oct. 22, 1999) which is expressly incorporated herein by reference in its entirety.